The present invention relates to a thread feed unit for circular knitting machines, in particular for double-cylinder machines for manufacturing socks and stockings.
Thread feed units in double-cylinder circular knitting machines are known which comprise a plurality of thread guides which are arranged at each feed station of the machine and are actuated by means of an actuation device which acts on said thread guides in order to move them individually, with their thread dispensing end, from a position which is close to the needle cylinders, in order to supply the thread to the needles which knit at the feed being considered, to a position which is spaced from the needle cylinder, in order to prevent the needles from engaging the thread, so as to be able to feed the needles with threads of different materials, counts or colors according to the various production requirements. The threads of the thread guides which are excluded from the knitting are retained by a cutting and clamping unit which is arranged laterally to the needle cylinder, releases the thread when the related thread guide is moved toward the needle cylinder, and cuts and clamps the thread when the related thread guide is spaced from the needle cylinder.
U.S. Pat. No. 4,233,826 granted Nov. 18, 1980 discloses an actuation device for a feed unit, or line-forming unit, for double-cylinder circular knitting machines wherein the movement for the approach or spacing of each thread guide of the line-forming unit is obtained by means of a cam which, by virtue of a lever, pulls or releases a bowden cable which acts on the related thread guide, causing its movement. The cam is actuated, by means of a gearwheel transmission, by a shaft which rotates at a rate which is synchronized with the rotation rate of the needle cylinder. Since the cam must be kept in such a position as to keep the related thread guide close to, or spaced from, the needle cylinders for the entire time required by the knitting, there is a pawl which connects the gearwheel transmission to the rotating shaft only for the time required to make the cam cover an arc of a circle which corresponds to a variation in the portion of its profile which acts, by means of the bowden cable, on the related thread guide.
The pawl is actuated by means of a system of levers which are mutually connected by another bowden cable and are actuated by a chain, or drum, which acts as programming element, since it is provided with protrusions which are arranged according to the knitting to be performed.
Although the actuation device described in the above mentioned patent has substantial advantages with respect to the actuation devices previously in use, subsequent technological progress in the field has pointed out limitations and problems in this device as well.
More particularly, in the continuous attempt to improve the quantity and quality of the production of these machines, in recent years there has been an increasing resort to electronically controlled actuators and accessories arranged around the needle cylinders, or in the region where the feed units are arranged.
Due to this reason, one of the fundamental problems in the design of these units is constituted by the containment of their bulk.
Furthermore, the use of cams with a preset profile for the actuation of the thread guides can constitute a limitation to operating flexibility, which is increasingly required in modern knitting machines.